Image encoding apparatus, image encoding method and program, image decoding apparatus, and image decoding method and program

1. An image decoding method to decode a block in a target image of a video sequence from encoded data, using a first vector representing a spatial relationship between the block to be decoded and a first prediction block in the target image, the image decoding method comprising: acquiring, from the encoded data, a prediction mode of the block to be decoded; in a case where the prediction mode is an intra block copy prediction mode, generating, for the block to be decoded in the target image, a candidate list of vectors associated with second blocks in the target image, each of the vectors in the candidate list representing spatial relationship between one of the second blocks and a corresponding block of second prediction blocks in the target image; adding, if a number of vectors in the candidate list is lower than a predetermined number, at least one vector having a zero vector value into the candidate list without adding a vector obtained from a third block in an image that is different from the target image, so that the number of vectors in the candidate list becomes the predetermined number, acquiring a vector as the first vector for the block to be decoded, selected from the candidate list corresponding to an index decoded from the encoded data; and determining the first prediction block for the block to be decoded from the decoded part of the target image, based on the acquired first vector, and wherein, in a case where the prediction mode is inter prediction mode, the candidate list is made up of vectors obtained from blocks in the target image having the block to be decoded, and blocks included in an image that is different from the target image, if available.

2. The method according to claim 1 , wherein the intra block copy prediction mode is a mode to decode a target block using an already-decoded image portion of an image including the target block.